1. Field of the Invention
The present invention relates to a battery module constituted by a plurality of prismatic cells connected adjacent each other, each of the cells accommodating therein an electrode plate group, comprising laminated positive and negative electrode plates with intervening separators therebetween, and particularly to the structure for fixing the electrode plate group in position within the cell.
2. Description of Related Art
Alkaline rechargeable batteries such as nickel metal hydride batteries are superior in energy density characteristics and have been used as the power source for an electric vehicle in recent years. Japanese Laid-Open Patent Application No. 7-161377 proposes a rechargeable battery, that is compact yet outputs a large amount of energy, in which a prismatic battery case accommodates therein, together with liquid electrolyte, positive and negative electrode plates stacked upon one another parallel to the long lateral walls of the battery case with intervening separators therebetween, the open end of the battery case being sealed with a lid member.
The battery disclosed in this publication is further described with reference to FIG. 4. Reference numeral 31 and 32 respectively denote a battery case and a cover, both made of polypropylene. An electrode plate group 33 is accommodated within the battery case 31 together with liquid electrolyte, and the open end of the battery case is sealed with the cover 32. The electrode plate group 33 comprises a plurality of negative electrode plates that are obtained by applying a paste mainly consisting of hydrogen-absorption alloy onto a flat plate core material, and a plurality of positive electrode plates that are obtained by packing powder mainly consisting of nickel hydroxide in a core material made of nickel porous material, these positive and negative electrode plates being laminated alternately with intervening separators made of polypropylene that is treated to obtain hydrophilicity. The positive electrode plates and the negative electrode plates are respectively connected to a positive and a negative electrode columns 35a, 35b through respective collector plates 34a, 34b, and the positive and negative electrode columns 35a, 35b are fixedly mounted to the cover 32. In this state, there is a gap t.sub.1 of about 2 to 10 mm between the bottom of the electrode plate group 33 and the inner bottom face of the battery case 31, and a gap t.sub.2 of about 1 to 5 mm between the lateral edges of the electrode plate group and the inner lateral face of the battery case.
These gaps t.sub.1, t.sub.2 are formed for the purpose of: 1) making the liquid distribution of electrolyte uniform, and 2) preventing the electrode plate group 33 from being compressed by the inner walls of the battery case 31 when the electrode plate 33 expands during the charging and discharging of the battery. This is because deformation of the electrode plate group 33 may lead to inefficient use of active material and deterioration of discharge capacity, rises in the internal pressure during charging, short-circuits, or peeling of active material from the electrode plates.
However, in the sealed prismatic battery described above, the electrode plate group 33 is fixed to the cover 32 through the electrode columns 35a, 35b. Accordingly, there are variations in the position of the electrode plate group 33 with respect to the battery case 31 due to the positional displacement between the battery case 31 and the cover 32 during welding, and the gaps t.sub.1, t.sub.2 are not uniformly formed, and electrode plates are sometimes deformed, leading to aforementioned various problems such as deterioration of discharge capacity due to insufficient use of active material, rises in the internal pressure during charging, and peeling of active material from the electrode plates. Further, there was the problem that the liquid electrolyte could not be distributed uniformly.